There has been provided an oil pump mounted in vehicles. The oil pump has an actuating chamber, a suction port, a delivery port, a suction passage for supplying oil to the suction port, a delivery passage to which the oil is delivered from the delivery port, a bypass passage for communicating with the delivery passage and the suction passage, and a rotor for actuating a pump action. Rotation of the rotor causes a pump action which sucks oil in the suction passage from the suction port so as to supply the oil to the delivery passage by way of the delivery port. When a flow amount of the oil is excessive in the delivery passage, a flow control valve sends the excessive oil in the delivery passage to the suction passage as a returning flow of oil by way of the bypass passage, thereby supplying the oil suitably from the delivery passage to a hydraulic apparatus.
By the way, when the excessive oil returns from the delivery passage exhibiting a high pressure to the suction passage exhibiting a low pressure by way of the bypass passage, the oil returns at a considerably high speed. Therefore, when the oil pump is used in an excessive long period, or when the oil pump is used in severe conditions, there is a possibility that corrosion portions occur by direct collision of the returning flow of oil in an inner wall surface of the bypass passage and the suction passage. The reason is assumed that corrosion occurs on the basis of cavitation. Especially, in the case where the oil pump is set to be a high pressure and a high capacity, the pressure is high in the delivery passage so that the oil returns at a considerable high speed. So, there is a possibility to generate corrosion. Further, in the case where the suction passage is formed of aluminum alloy, there is a possibility that corrosion occurs.
As the oil pump for improving corrosion problem, Japanese Unexamined Utility Model Publication 2-139386 discloses the technology which installed the shell body having a cylindrical shape formed of steel material having corrosion resistance at portions of the direct collision of the returning flow of oil. The technology can prevent corrosion at the portion of the direct collision of the returning flow of oil, even if the oil returns at a considerably high speed.
However, according to the above-mentioned technology of Publication 2-139386, the shell body formed of steel material having corrosion resistance has a cylindrical shape exhibiting a passage for oil-flow. The shell body shows a cylindrical shape continuing one circle in a circumferential direction of a center line of this passage in the cross section thereof, thereby requiring an abounding material having corrosion resistance. Also, this construction narrows the flow area of the cross section in the passage for returning oil. If the flow area is increased in the cross section of the passage for returning oil, there is a disadvantage in view of layout of the way and a wall thickness of the housing, etc. since the oil pump requires a small-size. So there is a limit in increasing a flow area of a passage for returning oil.
The present invention has been developed in view of the above-mentioned circumstances. It is an object of the present invention to provide an oil pump which can reduce a using amount of material having corrosion resistance and can ensure a flow area of a way for returning flow of oil while ensuring corrosion resistance in a portion of the direct collision of the returning flow of oil.